<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <title>User-Agent Client Hints</title>
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
	 vertical-align: baseline;
	 text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Reverse color scheme */
		color: black;
		border-color: #3980B5;
		border-bottom-width: 3px !important;
		margin-bottom: 0px !important;
	}
	.toc a:visited {
		border-color: #054572;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
		line-height: 1.1rem; /* consistent spacing */
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li             { font-weight: bold;   }
	.toc > li li          { font-weight: normal; }
	.toc > li li li       { font-size:   95%;    }
	.toc > li li li li    { font-size:   90%;    }
	.toc > li li li li .secno { font-size: 85%; }
	.toc > li li li li li { font-size:   85%;    }
	.toc > li li li li li .secno { font-size: 100%; }

	/* @supports not (display:grid) { */
		.toc > li             { margin: 1.5rem 0;    }
		.toc > li li          { margin: 0.3rem 0;    }
		.toc > li li li       { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}

		.toc li {
			clear: both;
		}

		:not(li) > .toc              { margin-left:  5rem; }
		.toc .secno                  { margin-left: -5rem; }
		.toc > li li li .secno       { margin-left: -7rem; }
		.toc > li li li li .secno    { margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc              { margin-left:  4rem; }
			.toc .secno                  { margin-left: -4rem; }
			.toc > li li li              { margin-left:  1rem; }
			.toc > li li li .secno       { margin-left: -5rem; }
			.toc > li li li li .secno    { margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			   comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
		}
		#toc .content:hover {
			background: rgba(75%, 75%, 75%, .25);
			border-bottom: 3px solid #054572;
			margin-bottom: -3px;
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index       { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li    { margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em }
	ul.index dl    { margin-top: 0; }
	ul.index dt    { margin: .2em 0 .2em 20px;}
	ul.index dd    { margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		background: #f7f8f9;
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}

/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good table positioning:
		   "content column" is 50ems wide at max; less on smaller screens.
		   Extra space (after ToC + content) is empty on the right.

		   1. When table < content column, centers table in column.
		   2. When content < table < available, left-aligns.
		   3. When table > available, fills available + scroll bar.
		*/ 
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			 * http://lea.verou.me/2012/04/background-attachment-local/
			 *
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
			            top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
			            top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
  <link href="https://www.w3.org/StyleSheets/TR/2016/cg-draft" rel="stylesheet">
  <link href="https://wicg.github.io/ua-client-hints/" rel="canonical">
<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}</style>
<style>/* style-selflinks */

.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: gray;
    color: white;
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: black;
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }</style>
<style>/* style-counters */

body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: #005a9c;
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
<style>/* style-dfn-panel */

.dfn-panel {
    position: absolute;
    z-index: 35;
    height: auto;
    width: -webkit-fit-content;
    width: fit-content;
    max-width: 300px;
    max-height: 500px;
    overflow: auto;
    padding: 0.5em 0.75em;
    font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
    background: #DDDDDD;
    color: black;
    border: outset 0.2em;
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: black; }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel ul { padding: 0; }
.dfn-panel li { list-style: inside; }
.dfn-panel.activated {
    display: inline-block;
    position: fixed;
    left: .5em;
    bottom: 2em;
    margin: 0 auto;
    max-width: calc(100vw - 1.5em - .4em - .5em);
    max-height: 30vh;
}

.dfn-paneled { cursor: pointer; }
</style>
<style>/* style-syntax-highlighting */
pre.idl.highlight { color: #708090; }
.highlight:not(.idl) { background: hsl(24, 20%, 95%); }
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">User-Agent Client Hints</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Draft Community Group Report, <time class="dt-updated" datetime="1970-01-01">1 January 1970</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wicg.github.io/ua-client-hints/">https://wicg.github.io/ua-client-hints/</a>
     <dt class="editor">Editor:
     <dd class="editor p-author h-card vcard" data-editor-id="56384"><a class="p-name fn u-email email" href="mailto:mkwst@google.com">Mike West</a> (<span class="p-org org">Google Inc.</span>)
     <dt>Participate:
     <dd><a href="https://github.com/WICG/ua-client-hints/issues/new">File an issue</a> (<a href="https://github.com/WICG/ua-client-hints/issues">open issues</a>)
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 1970 the Contributors to the User-Agent Client Hints Specification, published by the <a href="https://www.w3.org/community/wicg/">Web Platform Incubator Community Group</a> under the <a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement (CLA)</a>.
A human-readable <a href="http://www.w3.org/community/about/agreements/cla-deed/">summary</a> is available. </p>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This document defines a set of Client Hints that aim to provide developers with the ability to

    perform agent-based content negotiation when necessary, while avoiding the historical baggage and
    passive fingerprinting surface exposed by the venerable `User-Agent` header.</p>
  </div>
  <div data-fill-with="at-risk"></div>
  <h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
  <div data-fill-with="status">
   <p> This specification was published by the <a href="https://www.w3.org/community/wicg/">Web Platform Incubator Community Group</a>.
  It is not a W3C Standard nor is it on the W3C Standards Track.

  Please note that under the <a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement (CLA)</a> there is a limited opt-out and other conditions apply.

  Learn more about <a href="http://www.w3.org/community/">W3C Community and Business Groups</a>. </p>
   <p></p>
  </div>
  <div data-fill-with="at-risk"></div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
     <ol class="toc">
      <li><a href="#examples"><span class="secno">1.1</span> <span class="content">Examples</span></a>
     </ol>
    <li>
     <a href="#http-ua-hints"><span class="secno">2</span> <span class="content">User Agent Hints</span></a>
     <ol class="toc">
      <li><a href="#sec-ch-arch"><span class="secno">2.1</span> <span class="content">The 'Sec-CH-Arch' Header Field</span></a>
      <li><a href="#sec-ch-model"><span class="secno">2.2</span> <span class="content">The 'Sec-CH-Model' Header Field</span></a>
      <li><a href="#sec-ch-platform"><span class="secno">2.3</span> <span class="content">The 'Sec-CH-Platform' Header Field</span></a>
      <li><a href="#sec-ch-ua"><span class="secno">2.4</span> <span class="content">The 'Sec-CH-UA' Header Field</span></a>
      <li><a href="#sec-ch-mobile"><span class="secno">2.5</span> <span class="content">The 'Sec-CH-Mobile' Header Field</span></a>
      <li><a href="#fetch-integration"><span class="secno">2.6</span> <span class="content">Integration with Fetch</span></a>
     </ol>
    <li>
     <a href="#interface"><span class="secno">3</span> <span class="content">Interface</span></a>
     <ol class="toc">
      <li><a href="#processing"><span class="secno">3.1</span> <span class="content">Processing model</span></a>
     </ol>
    <li>
     <a href="#security-privacy"><span class="secno">4</span> <span class="content">Security and Privacy Considerations</span></a>
     <ol class="toc">
      <li><a href="#secure-transport"><span class="secno">4.1</span> <span class="content">Secure Transport</span></a>
      <li><a href="#delegation"><span class="secno">4.2</span> <span class="content">Delegation</span></a>
      <li><a href="#access"><span class="secno">4.3</span> <span class="content">Access Restrictions</span></a>
     </ol>
    <li>
     <a href="#impl-considerations"><span class="secno">5</span> <span class="content">Implementation Considerations</span></a>
     <ol class="toc">
      <li><a href="#user-agent"><span class="secno">5.1</span> <span class="content">The 'User-Agent' Header</span></a>
      <li><a href="#grease"><span class="secno">5.2</span> <span class="content">GREASE-like UA Strings</span></a>
      <li><a href="#sec-ch"><span class="secno">5.3</span> <span class="content">The 'Sec-CH-' prefix</span></a>
     </ol>
    <li>
     <a href="#iana"><span class="secno">6</span> <span class="content">IANA Considerations</span></a>
     <ol class="toc">
      <li><a href="#iana-arch"><span class="secno">6.1</span> <span class="content">'Sec-CH-Arch' Header Field</span></a>
      <li><a href="#iana-model"><span class="secno">6.2</span> <span class="content">'Sec-CH-Model' Header Field</span></a>
      <li><a href="#iana-platform"><span class="secno">6.3</span> <span class="content">'Sec-CH-Platform' Header Field</span></a>
      <li><a href="#iana-ua"><span class="secno">6.4</span> <span class="content">'Sec-CH-UA' Header Field</span></a>
      <li><a href="#iana-mobile"><span class="secno">6.5</span> <span class="content">'Sec-CH-Mobile' Header Field</span></a>
      <li><a href="#iana-user-agent"><span class="secno">6.6</span> <span class="content">'User-Agent' Header Field</span></a>
     </ol>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
      <li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
    <li><a href="#idl-index"><span class="secno"></span> <span class="content">IDL Index</span></a>
    <li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>Today, user agents generally identify themselves to servers by sending a <code>User-Agent</code> HTTP request
header field along with each request (defined in Section 5.5.3 of <a data-link-type="biblio" href="#biblio-rfc7231">[RFC7231]</a>). Ideally, this header
would give servers the ability to perform content negotiation, sending down exactly those bits that
best represent the requested resource in a given user agent, optimizing both bandwidth and user
experience. In practice, however, this header’s value exposes far more information about the user’s
device than seems appropriate as a default, on the one hand, and intentionally obscures the true
user agent in order to bypass misguided server-side heuristics, on the other.</p>
   <p>For example, a recent version of Chrome on iOS identifies itself as:</p>
<pre class="language-http highlight">  User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 12_0 like Mac OS X)
              AppleWebKit/605.1.15 (KHTML, like Gecko)
              CriOS/69.0.3497.105 Mobile/15E148 Safari/605.1
</pre>
   <p>While a recent version of Edge identifies itself as:</p>
<pre class="language-http highlight">  User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64)
              AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.2704.79
              Safari/537.36 Edge/18.014
</pre>
   <p>There’s quite a bit of information packed into those strings (along with a fair number of lies).
Version numbers, platform details, model information, etc. are all broadcast along with every
request, and form the basis for fingerprinting schemes of all sorts. Individual vendors have taken
stabs at altering their user agent strings, and have run into a few categories of feedback from
developers that have stymied historical approaches:</p>
   <ol>
    <li data-md>
     <p>Brand and version information (e.g. "Chrome 69") allows websites to work around known bugs in
specific releases that aren’t otherwise detectable. For example, implementations of Content
Security Policy have varied wildly between vendors, and it’s difficult to know what policy to
send in an HTTP response without knowing what browser is responsible for its parsing and
execution.</p>
    <li data-md>
     <p>Developers will often negotiate what content to send based on the user agent and platform. Some
application frameworks, for instance, will style an application on iOS differently from the same
application on Android in order to match each platform’s aesthetic and design patterns.</p>
    <li data-md>
     <p>Similarly to #1, OS revisions and architecture can be responsible for specific bugs which can
be worked around in website’s code, and narrowly useful for things like selecting appropriate
executables for download (32 vs 64 bit, ARM vs Intel, etc).</p>
    <li data-md>
     <p>Sophisticated developers use model/make to tailor their sites to the capabilities of the
device (e.g. <a data-link-type="biblio" href="#biblio-facebookyearclass">[FacebookYearClass]</a>) and to pinpoint performance bugs and regressions which
sometimes are specific to model/make.</p>
   </ol>
   <p>This document proposes a mechanism which might allow user agents to be a bit more aggressive about
removing entropy from the <code>User-Agent</code> string generally by giving servers that really need some
specific details about the client the ability to opt-into receiving them. It introduces four new
Client Hints (<a data-link-type="biblio" href="#biblio-i-dietf-httpbis-client-hints">[I-D.ietf-httpbis-client-hints]</a>) that can provide the client’s branding and version
information, the underlying operating system’s branding and major version, as well as details about
the underlying device. Rather than broadcasting this data to everyone, all the time, user agents can
make reasonable decisions about how to respond to given sites' requests for more granular data,
reducing the passive fingerprinting surface area exposed to the network.</p>
   <h3 class="heading settled" data-level="1.1" id="examples"><span class="secno">1.1. </span><span class="content">Examples</span><a class="self-link" href="#examples"></a></h3>
   <p>A user navigates to <code>https://example.com/</code> for the first time. Their user agent sends the following
header along with the HTTP request:</p>
<pre class="language-http highlight">  Sec-CH-UA: "Examplary Browser 73"
</pre>
   <p>The server is interested in rendering content consistent with the user’s underlying platform, and
asks for a little more information by sending an <code>Accept-CH</code> header (Section 2.2.1 of <a data-link-type="biblio" href="#biblio-i-dietf-httpbis-client-hints">[I-D.ietf-httpbis-client-hints]</a>) along with the initial response:</p>
<pre class="language-http highlight">  Accept-CH: UA, Platform
</pre>
   <p>In response, the user agent includes more detailed version information, as well as information about
the underlying platform in the next request:</p>
<pre class="language-http highlight">  Sec-CH-UA: "Examplary Browser 73.3R8.2H.1"
  Sec-CH-Platform: "Windows 10"
</pre>
   <h2 class="heading settled" data-level="2" id="http-ua-hints"><span class="secno">2. </span><span class="content">User Agent Hints</span><a class="self-link" href="#http-ua-hints"></a></h2>
   <p>The following sections define a number of HTTP request header fields that expose detail about a
given user agent, which servers can opt-into receiving via the Client Hints infrastructure defined
in <a data-link-type="biblio" href="#biblio-i-dietf-httpbis-client-hints">[I-D.ietf-httpbis-client-hints]</a>. The definitions below assume that each user agent has defined
a number of properties for itself:</p>
   <ul>
    <li data-md>
     <p><dfn class="dfn-paneled" data-dfn-for="user agent" data-dfn-type="dfn" data-export id="user-agent-brand">brand</dfn> (for example: "cURL", "Edge", "The World’s Best Web Browser")</p>
    <li data-md>
     <p><dfn class="dfn-paneled" data-dfn-for="user agent" data-dfn-type="dfn" data-export id="user-agent-major-version">major version</dfn> (for example: "72", "3", or "28")</p>
    <li data-md>
     <p><dfn class="dfn-paneled" data-dfn-for="user agent" data-dfn-type="dfn" data-export id="user-agent-full-version">full version</dfn> (for example: "72.0.3245.12", "3.14159", or "297.70E04154A")</p>
    <li data-md>
     <p><dfn class="dfn-paneled" data-dfn-for="user agent" data-dfn-type="dfn" data-export id="user-agent-platform-brand-and-version">platform brand and version</dfn> (for example: "Windows NT 6.0", "iOS 15", or "AmazingOS 17G")</p>
    <li data-md>
     <p><dfn class="dfn-paneled" data-dfn-for="user agent" data-dfn-type="dfn" data-export id="user-agent-platform-architecture">platform architecture</dfn> (for example: "ARM64", or "ia32")</p>
    <li data-md>
     <p><dfn class="dfn-paneled" data-dfn-for="user agent" data-dfn-type="dfn" data-export id="user-agent-model">model</dfn> (for example: "", or "Pixel 2 XL")</p>
    <li data-md>
     <p><dfn class="dfn-paneled" data-dfn-for="user agent" data-dfn-type="dfn" data-export id="user-agent-mobileness">mobileness</dfn> (for example: ?0 or ?1)</p>
   </ul>
   <p>User agents SHOULD keep these strings short and to the point, but servers MUST accept arbitrary
values for each, as they are all values constructed at the user agent’s whim.</p>
   <h3 class="heading settled" data-level="2.1" id="sec-ch-arch"><span class="secno">2.1. </span><span class="content">The 'Sec-CH-Arch' Header Field</span><a class="self-link" href="#sec-ch-arch"></a></h3>
   <p>The <dfn data-dfn-type="http-header" data-export id="http-headerdef-sec-ch-arch"><code>Sec-CH-Arch</code><a class="self-link" href="#http-headerdef-sec-ch-arch"></a></dfn> request header field gives a server information about
the architecture of the platform on which a given user agent is executing. It is a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-">Structured Header</a> whose value MUST be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.3.3" id="ref-for-section-3.3.3">string</a> <a data-link-type="biblio" href="#biblio-i-dietf-httpbis-header-structure">[I-D.ietf-httpbis-header-structure]</a>.</p>
   <p>The header’s ABNF is:</p>
<pre class="language-abnf highlight">  <c- nc>Sec-CH-Arch</c-> <c- o>=</c-> <c- nc>sh-string</c->
</pre>
   <p>To <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export data-local-lt="set-arch" id="abstract-opdef-set-the-sec-ch-arch-header-for-a-request">set the <code>Sec-CH-Arch</code> header for a request</dfn>, given a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request" id="ref-for-concept-request">request</a> (<var>r</var>),
user agents MUST:</p>
   <ol>
    <li data-md>
     <p>If <var>r</var>’s <a data-link-type="dfn" href="#request-client-hints-set" id="ref-for-request-client-hints-set">client-hints set</a> <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-contain" id="ref-for-list-contain">contains</a> <code>Arch</code>, then:</p>
     <ol>
      <li data-md>
       <p>Let <code>value</code> be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-①">Structured Header</a> object whose value is the user agent’s <a data-link-type="dfn" href="#user-agent-platform-architecture" id="ref-for-user-agent-platform-architecture">platform architecture</a>.</p>
      <li data-md>
       <p><a data-link-type="dfn">Set a structured header</a> in <var>r</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list">header list</a> whose name is <code>Sec-CH-Arch</code>, and whose value is <code>value</code>.</p>
     </ol>
   </ol>
   <h3 class="heading settled" data-level="2.2" id="sec-ch-model"><span class="secno">2.2. </span><span class="content">The 'Sec-CH-Model' Header Field</span><a class="self-link" href="#sec-ch-model"></a></h3>
   <p>The <dfn data-dfn-type="http-header" data-export id="http-headerdef-sec-ch-model"><code>Sec-CH-Model</code><a class="self-link" href="#http-headerdef-sec-ch-model"></a></dfn> request header field gives a server information about
the device on which a given user agent is executing. It is a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-②">Structured Header</a> whose value MUST
be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.3.3" id="ref-for-section-3.3.3①">string</a> <a data-link-type="biblio" href="#biblio-i-dietf-httpbis-header-structure">[I-D.ietf-httpbis-header-structure]</a>.</p>
   <p>The header’s ABNF is:</p>
<pre class="language-abnf highlight">  <c- nc>Sec-CH-Model</c-> <c- o>=</c-> <c- nc>sh-string</c->
</pre>
   <p>To <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export data-local-lt="set-model" id="abstract-opdef-set-the-sec-ch-model-header-for-a-request">set the <code>Sec-CH-Model</code> header for a request</dfn>, given a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request" id="ref-for-concept-request①">request</a> (<var>r</var>),
user agents MUST:</p>
   <ol>
    <li data-md>
     <p>If <var>r</var>’s <a data-link-type="dfn" href="#request-client-hints-set" id="ref-for-request-client-hints-set①">client-hints set</a> <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-contain" id="ref-for-list-contain①">contains</a> <code>Model</code>, then:</p>
     <ol>
      <li data-md>
       <p>Let <code>value</code> be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-③">Structured Header</a> object whose value is the user agent’s <a data-link-type="dfn" href="#user-agent-model" id="ref-for-user-agent-model">model</a>.</p>
      <li data-md>
       <p><a data-link-type="dfn">Set a structured header</a> in <var>r</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list①">header list</a> whose name is <code>Sec-CH-Model</code>, and whose value is <code>value</code>.</p>
     </ol>
   </ol>
   <p class="issue" id="issue-a1588654"><a class="self-link" href="#issue-a1588654"></a> Perhaps <code>Sec-CH-Mobile</code> is enough, and we don’t need to expose the model?</p>
   <h3 class="heading settled" data-level="2.3" id="sec-ch-platform"><span class="secno">2.3. </span><span class="content">The 'Sec-CH-Platform' Header Field</span><a class="self-link" href="#sec-ch-platform"></a></h3>
   <p>The <dfn data-dfn-type="http-header" data-export id="http-headerdef-sec-ch-platform"><code>Sec-CH-Platform</code><a class="self-link" href="#http-headerdef-sec-ch-platform"></a></dfn> request header field gives a server information about
the platform on which a given user agent is executing. It is a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-④">Structured Header</a> whose value
MUST be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.3.3" id="ref-for-section-3.3.3②">string</a> <a data-link-type="biblio" href="#biblio-i-dietf-httpbis-header-structure">[I-D.ietf-httpbis-header-structure]</a>.</p>
   <p>The header’s ABNF is:</p>
<pre class="language-abnf highlight">  <c- nc>Sec-CH-Platform</c-> <c- o>=</c-> <c- nc>sh-string</c->
</pre>
   <p>To <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export data-local-lt="set-platform" id="abstract-opdef-set-the-sec-ch-platform-header-for-a-request">set the <code>Sec-CH-Platform</code> header for a request</dfn>, given a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request" id="ref-for-concept-request②">request</a> (<var>r</var>),
user agents MUST:</p>
   <ol>
    <li data-md>
     <p>If <var>r</var>’s <a data-link-type="dfn" href="#request-client-hints-set" id="ref-for-request-client-hints-set②">client-hints set</a> <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-contain" id="ref-for-list-contain②">contains</a> <code>Platform</code>, then:</p>
     <ol>
      <li data-md>
       <p>Let <code>value</code> be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-⑤">Structured Header</a> object whose value is the user agent’s <a data-link-type="dfn" href="#user-agent-platform-brand-and-version" id="ref-for-user-agent-platform-brand-and-version">platform brand and version</a>.</p>
      <li data-md>
       <p><a data-link-type="dfn">Set a structured header</a> in <var>r</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list②">header list</a> whose name is <code>Sec-CH-Platform</code>, and whose value is <code>value</code>.</p>
     </ol>
   </ol>
   <h3 class="heading settled" data-level="2.4" id="sec-ch-ua"><span class="secno">2.4. </span><span class="content">The 'Sec-CH-UA' Header Field</span><a class="self-link" href="#sec-ch-ua"></a></h3>
   <p>The <dfn data-dfn-type="http-header" data-export id="http-headerdef-sec-ch-ua"><code>Sec-CH-UA</code><a class="self-link" href="#http-headerdef-sec-ch-ua"></a></dfn> request header field gives a server information about a
user agent’s branding and version. It is a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-⑥">Structured Header</a> whose value MUST be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.1" id="ref-for-section-3.1">list</a> <a data-link-type="biblio" href="#biblio-i-dietf-httpbis-header-structure">[I-D.ietf-httpbis-header-structure]</a>.</p>
   <p>The header’s ABNF is:</p>
<pre class="language-abnf highlight">  <c- nc>Sec-CH-UA</c-> <c- o>=</c-> <c- nc>sh-list</c->
</pre>
   <p>Unlike most Client Hints, the <code>Sec-CH-UA</code> header will be sent with all requests, whether or not the
server opted-into receiving the header via an <code>Accept-CH</code> header. Prior to an opt-in, however, it
will include only the user agent’s branding information, and the major version number (both of which
are fairly clearly sniffable by "examining the structure of other headers and by testing for the
availability and semantics of the features introduced or modified between releases of a particular
browser" <a data-link-type="biblio" href="#biblio-janc2014">[Janc2014]</a>).</p>
   <p>To <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export data-local-lt="set-ua" id="abstract-opdef-set-the-sec-ch-ua-header-for-a-request">set the <code>Sec-CH-UA</code> header for a request</dfn>, given a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request" id="ref-for-concept-request③">request</a> (<var>r</var>),
user agents MUST:</p>
   <ol>
    <li data-md>
     <p>Let <var>value</var> be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-⑦">Structured Header</a> object whose value is a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.1" id="ref-for-section-3.1①">list</a>.</p>
    <li data-md>
     <p>Let <var>version</var> be the user agent’s <a data-link-type="dfn" href="#user-agent-full-version" id="ref-for-user-agent-full-version">full version</a> if <var>r</var>’s <a data-link-type="dfn" href="#request-client-hints-set" id="ref-for-request-client-hints-set③">client-hints set</a> <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-contain" id="ref-for-list-contain③">contains</a> <code>UA</code>, and the user agent’s <a data-link-type="dfn" href="#user-agent-major-version" id="ref-for-user-agent-major-version">major version</a> otherwise.</p>
    <li data-md>
     <p>Let <var>ua</var> be a string whose value is the <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#string-concatenate" id="ref-for-string-concatenate">concatenation</a> of the user agent’s <a data-link-type="dfn" href="#user-agent-brand" id="ref-for-user-agent-brand">brand</a>, a U+0020 SPACE character, and <var>version</var>.</p>
     <p class="issue" id="issue-af2ff260"><a class="self-link" href="#issue-af2ff260"></a> Should we split the version out into a separate <code>Sec-CH-UA-Version</code> header? Or keep it here? <a href="https://github.com/wicg/ua-client-hints/issues/7">&lt;https://github.com/wicg/ua-client-hints/issues/7></a></p>
    <li data-md>
     <p><a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-append" id="ref-for-list-append">Append</a> <var>ua</var> to <var>value</var>.</p>
    <li data-md>
     <p>The user agent MAY execute the following steps:</p>
     <ol>
      <li data-md>
       <p><a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-append" id="ref-for-list-append①">Append</a> additional items to <var>value</var> containing arbitrary brand and version
combinations.</p>
      <li data-md>
       <p>Randomize the order of the items in <var>value</var>.</p>
     </ol>
     <p class="note" role="note"><span>Note:</span> See <a href="#grease">§ 5.2 GREASE-like UA Strings</a> for more details on why these steps might be appropriate.</p>
    <li data-md>
     <p><a data-link-type="dfn">Set a structured header</a> in <var>r</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list③">header list</a> whose name is <code>Sec-CH-UA</code>, and whose value is <var>value</var>.</p>
   </ol>
   <h3 class="heading settled" data-level="2.5" id="sec-ch-mobile"><span class="secno">2.5. </span><span class="content">The 'Sec-CH-Mobile' Header Field</span><a class="self-link" href="#sec-ch-mobile"></a></h3>
   <p>The <dfn data-dfn-type="http-header" data-export id="http-headerdef-sec-ch-mobile"><code>Sec-CH-Mobile</code><a class="self-link" href="#http-headerdef-sec-ch-mobile"></a></dfn> request header field gives a server information about
whether or not a user agent prefers a "mobile" user experience. It is a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-⑧">Structured Header</a> whose value MUST be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.3.4" id="ref-for-section-3.3.4">boolean</a> <a data-link-type="biblio" href="#biblio-i-dietf-httpbis-header-structure">[I-D.ietf-httpbis-header-structure]</a>.</p>
   <p>The header’s ABNF is:</p>
<pre class="language-abnf highlight">  <c- nc>Sec-CH-Mobile</c-> <c- o>=</c-> <c- nc>sh-boolean</c->
</pre>
   <p>To <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export data-local-lt="set-mobile" id="abstract-opdef-set-the-sec-ch-mobile-header-for-a-request">set the <code>Sec-CH-Mobile</code> header for a request</dfn>, given a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request" id="ref-for-concept-request④">request</a> (<var>r</var>),
user agents MUST:</p>
   <ol>
    <li data-md>
     <p>If the <var>r</var>’s <a data-link-type="dfn" href="#request-client-hints-set" id="ref-for-request-client-hints-set④">client-hints set</a> <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-contain" id="ref-for-list-contain④">contains</a> <code>Mobile</code>, then:</p>
     <ol>
      <li data-md>
       <p>Let <code>value</code> be a <a data-link-type="dfn" href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#" id="termref-for-⑨">Structured Header</a> whose value is the user agent’s <a data-link-type="dfn" href="#user-agent-mobileness" id="ref-for-user-agent-mobileness">mobileness</a>.</p>
      <li data-md>
       <p><a data-link-type="dfn">Set a structured header</a> in <var>r</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list④">header list</a> whose name is <code>Sec-CH-Mobile</code>, and whose value is <code>value</code>.</p>
     </ol>
   </ol>
   <h3 class="heading settled" data-level="2.6" id="fetch-integration"><span class="secno">2.6. </span><span class="content">Integration with Fetch</span><a class="self-link" href="#fetch-integration"></a></h3>
   <p>The Fetch specification should call into the following algorithm in place of the current Step 5.11
in its HTTP-network-or-cache fetch algorithm.</p>
   <p>To <dfn data-dfn-type="abstract-op" data-export id="abstract-opdef-set-the-user-agent-metadata-for-a-request">set the user agent metadata for a request<a class="self-link" href="#abstract-opdef-set-the-user-agent-metadata-for-a-request"></a></dfn> (<var>r</var>), the user agent MUST
execute the following steps:</p>
   <ol>
    <li data-md>
     <p>If <var>r</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list⑤">header list</a> does not <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#header-list-contains" id="ref-for-header-list-contains">contain</a> <code>User-Agent</code>, then the user
agent MAY append <code>User-Agent</code>/[=default <code>User-Agent</code> value=] to <var>r</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list⑥">header list</a>.</p>
    <li data-md>
     <p><a data-link-type="abstract-op" href="#abstract-opdef-set-the-sec-ch-arch-header-for-a-request" id="ref-for-abstract-opdef-set-the-sec-ch-arch-header-for-a-request">Set the <code>Sec-CH-Arch</code> header for a request</a>, given <var>r</var>.</p>
    <li data-md>
     <p><a data-link-type="abstract-op" href="#abstract-opdef-set-the-sec-ch-mobile-header-for-a-request" id="ref-for-abstract-opdef-set-the-sec-ch-mobile-header-for-a-request">Set the <code>Sec-CH-Mobile</code> header for a request</a>, given <var>r</var>.</p>
    <li data-md>
     <p><a data-link-type="abstract-op" href="#abstract-opdef-set-the-sec-ch-model-header-for-a-request" id="ref-for-abstract-opdef-set-the-sec-ch-model-header-for-a-request">Set the <code>Sec-CH-Model</code> header for a request</a>, given <var>r</var>.</p>
    <li data-md>
     <p><a data-link-type="abstract-op" href="#abstract-opdef-set-the-sec-ch-platform-header-for-a-request" id="ref-for-abstract-opdef-set-the-sec-ch-platform-header-for-a-request">Set the <code>Sec-CH-Platform</code> header for a request</a>, given <var>r</var>.</p>
    <li data-md>
     <p><a data-link-type="abstract-op" href="#abstract-opdef-set-the-sec-ch-ua-header-for-a-request" id="ref-for-abstract-opdef-set-the-sec-ch-ua-header-for-a-request">Set the <code>Sec-CH-UA</code> header for a request</a>, given <var>r</var>.</p>
   </ol>
   <h2 class="heading settled" data-level="3" id="interface"><span class="secno">3. </span><span class="content">Interface</span><a class="self-link" href="#interface"></a></h2>
<pre class="idl highlight def">[<a class="idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#Exposed" id="ref-for-Exposed"><c- g>Exposed</c-></a>=<c- n>Window</c->]
<c- b>interface</c-> <dfn class="dfn-paneled idl-code" data-dfn-type="interface" data-export id="navigatoruadata"><code><c- g>NavigatorUAData</c-></code></dfn> {
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString"><c- b>DOMString</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="NavigatorUAData" data-dfn-type="attribute" data-export data-readonly data-type="DOMString" id="dom-navigatoruadata-brand"><code><c- g>brand</c-></code></dfn>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString①"><c- b>DOMString</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="NavigatorUAData" data-dfn-type="attribute" data-export data-readonly data-type="DOMString" id="dom-navigatoruadata-version"><code><c- g>version</c-></code></dfn>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString②"><c- b>DOMString</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="NavigatorUAData" data-dfn-type="attribute" data-export data-readonly data-type="DOMString" id="dom-navigatoruadata-platform"><code><c- g>platform</c-></code></dfn>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString③"><c- b>DOMString</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="NavigatorUAData" data-dfn-type="attribute" data-export data-readonly data-type="DOMString" id="dom-navigatoruadata-architecture"><code><c- g>architecture</c-></code></dfn>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString④"><c- b>DOMString</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="NavigatorUAData" data-dfn-type="attribute" data-export data-readonly data-type="DOMString" id="dom-navigatoruadata-model"><code><c- g>model</c-></code></dfn>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-boolean" id="ref-for-idl-boolean"><c- b>boolean</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="NavigatorUAData" data-dfn-type="attribute" data-export data-readonly data-type="boolean" id="dom-navigatoruadata-mobile"><code><c- g>mobile</c-></code></dfn>;
};
<c- b>interface</c-> <c- b>mixin</c-> <dfn class="dfn-paneled idl-code" data-dfn-type="interface" data-export id="navigatorua"><code><c- g>NavigatorUA</c-></code></dfn> {
  [<a class="idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#SecureContext" id="ref-for-SecureContext"><c- g>SecureContext</c-></a>] <c- b>Promise</c->&lt;<a class="n" data-link-type="idl-name" href="#navigatoruadata" id="ref-for-navigatoruadata"><c- n>NavigatorUAData</c-></a>> <a class="idl-code" data-link-type="method" href="#dom-navigatorua-getuseragent" id="ref-for-dom-navigatorua-getuseragent"><c- g>getUserAgent</c-></a>();
};
<a class="n" data-link-type="idl-name" href="https://html.spec.whatwg.org/multipage/system-state.html#navigator" id="ref-for-navigator"><c- n>Navigator</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="#navigatorua" id="ref-for-navigatorua"><c- n>NavigatorUA</c-></a>;

</pre>
   <h3 class="heading settled" data-level="3.1" id="processing"><span class="secno">3.1. </span><span class="content">Processing model</span><a class="self-link" href="#processing"></a></h3>
    <dfn class="dfn-paneled idl-code" data-dfn-for="NavigatorUA" data-dfn-type="method" data-export id="dom-navigatorua-getuseragent"><code>getUserAgent()</code></dfn> method MUST run these steps: 
   <ol>
    <li data-md>
     <p>Let <var>p</var> be a <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide/#a-new-promise" id="ref-for-a-new-promise">a new promise</a>.</p>
    <li data-md>
     <p>Run the following steps <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel" id="ref-for-in-parallel">in parallel</a>:</p>
     <ol>
      <li data-md>
       <p>Let <var>UAData</var> be a new <code class="idl"><a data-link-type="idl" href="#navigatoruadata" id="ref-for-navigatoruadata①">NavigatorUAData</a></code> object whose values are initialized as follows:</p>
       <dl>
        <dt data-md><code class="idl"><a data-link-type="idl" href="#dom-navigatoruadata-brand" id="ref-for-dom-navigatoruadata-brand">brand</a></code>
        <dd data-md>
         <p>The user agent’s <a data-link-type="dfn" href="#user-agent-brand" id="ref-for-user-agent-brand①">brand</a>.</p>
        <dt data-md><code class="idl"><a data-link-type="idl" href="#dom-navigatoruadata-platform" id="ref-for-dom-navigatoruadata-platform">platform</a></code>
        <dd data-md>
         <p>The user agent’s <a data-link-type="dfn" href="#user-agent-platform-brand-and-version" id="ref-for-user-agent-platform-brand-and-version①">platform brand and version</a>.</p>
        <dt data-md><code class="idl"><a data-link-type="idl" href="#dom-navigatoruadata-architecture" id="ref-for-dom-navigatoruadata-architecture">architecture</a></code>
        <dd data-md>
         <p>The user agent’s <a data-link-type="dfn" href="#user-agent-platform-architecture" id="ref-for-user-agent-platform-architecture①">platform architecture</a>.</p>
        <dt data-md><code class="idl"><a data-link-type="idl" href="#dom-navigatoruadata-model" id="ref-for-dom-navigatoruadata-model">model</a></code>
        <dd data-md>
         <p>The user agent’s <a data-link-type="dfn" href="#user-agent-model" id="ref-for-user-agent-model①">model</a>.</p>
        <dt data-md><code class="idl"><a data-link-type="idl" href="#dom-navigatoruadata-mobile" id="ref-for-dom-navigatoruadata-mobile">mobile</a></code>
        <dd data-md>
         <p>The user agent’s <a data-link-type="dfn" href="#user-agent-mobileness" id="ref-for-user-agent-mobileness①">mobileness</a>.</p>
        <dt data-md><code class="idl"><a data-link-type="idl" href="#dom-navigatoruadata-version" id="ref-for-dom-navigatoruadata-version">version</a></code>
        <dd data-md>
         <p>The user agent’s <a data-link-type="dfn" href="#user-agent-full-version" id="ref-for-user-agent-full-version①">full version</a>.</p>
       </dl>
      <li data-md>
       <p><a data-link-type="dfn">Resolve</a> <var>p</var> with <var>UAData</var>.</p>
     </ol>
    <li data-md>
     <p>Return <var>p</var>.</p>
   </ol>
   <p class="issue" id="issue-611030b9"><a class="self-link" href="#issue-611030b9"></a> Provide a method to only access the UA’s major version.</p>
   <h2 class="heading settled" data-level="4" id="security-privacy"><span class="secno">4. </span><span class="content">Security and Privacy Considerations</span><a class="self-link" href="#security-privacy"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="secure-transport"><span class="secno">4.1. </span><span class="content">Secure Transport</span><a class="self-link" href="#secure-transport"></a></h3>
   <p>Client Hints will not be delivered to non-secure endpoints (see the secure transport requirements in
Section 2.2.1 of <a data-link-type="biblio" href="#biblio-i-dietf-httpbis-client-hints">[I-D.ietf-httpbis-client-hints]</a>). This means that user agent information will not
be leaked over plaintext channels, reducing the opportunity for network attackers to build a profile
of a given agent’s behavior over time.</p>
   <h3 class="heading settled" data-level="4.2" id="delegation"><span class="secno">4.2. </span><span class="content">Delegation</span><a class="self-link" href="#delegation"></a></h3>
   <p>Client Hints will be delegated from top-level pages via Feature Policy (once a few patches against
Fetch and Client Hints and Feature Policy land. This reduces the likelihood that user agent
information will be delivered along with subresource requests, which reduces the potential for
passive fingerprinting. The following issues cover the dependencies:</p>
   <p class="issue" id="issue-b875ecdd"><a class="self-link" href="#issue-b875ecdd"></a> Fetch integration of Accept-CH opt-in, and the definition of a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request" id="ref-for-concept-request⑤">request</a>'s <dfn class="dfn-paneled" data-dfn-for="request" data-dfn-type="dfn" data-noexport id="request-client-hints-set">client-hints set</dfn>. <a href="https://github.com/whatwg/fetch/issues/773">&lt;https://github.com/whatwg/fetch/issues/773></a></p>
   <p class="issue" id="issue-ff94095d"><a class="self-link" href="#issue-ff94095d"></a> HTML integration of Accept-CH-Lifetime and the ACHL cache. <a href="https://github.com/whatwg/html/issues/3774">&lt;https://github.com/whatwg/html/issues/3774></a></p>
   <p class="issue" id="issue-ad33bb81"><a class="self-link" href="#issue-ad33bb81"></a> Adding new CH features to the CH list in Fetch <a href="https://github.com/whatwg/fetch/issues/725">&lt;https://github.com/whatwg/fetch/issues/725></a></p>
   <p class="issue" id="issue-4794e88a"><a class="self-link" href="#issue-4794e88a"></a> 3rd-party opt-in <a href="https://github.com/whatwg/fetch/issues/811">&lt;https://github.com/whatwg/fetch/issues/811></a></p>
   <p class="issue" id="issue-4794e88a①"><a class="self-link" href="#issue-4794e88a①"></a> 3rd-party opt-in <a href="https://github.com/wicg/feature-policy/issues/220">&lt;https://github.com/wicg/feature-policy/issues/220></a></p>
   <p class="issue" id="issue-4b5acc88"><a class="self-link" href="#issue-4b5acc88"></a> These are all out of date. Yoav will fix them.</p>
   <h3 class="heading settled" data-level="4.3" id="access"><span class="secno">4.3. </span><span class="content">Access Restrictions</span><a class="self-link" href="#access"></a></h3>
   <p>The information in the Client Hints defined above reveals quite a bit of information about the user
agent and the platform/device upon which it runs. User agents ought to exercise judgement before
granting access to this information, and MAY impose restrictions above and beyond the secure
transport and delegation requirements noted above. For instance, user agents could choose to reveal <a data-link-type="dfn" href="#user-agent-platform-architecture" id="ref-for-user-agent-platform-architecture②">platform architecture</a> only on requests it intends to download, giving the server the
opportunity to serve the right binary. Likewise, they could offer users control over the values
revealed to servers, or gate access on explicit user interaction via a permission prompt or via a
settings interface.</p>
   <h2 class="heading settled" data-level="5" id="impl-considerations"><span class="secno">5. </span><span class="content">Implementation Considerations</span><a class="self-link" href="#impl-considerations"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="user-agent"><span class="secno">5.1. </span><span class="content">The 'User-Agent' Header</span><a class="self-link" href="#user-agent"></a></h3>
   <p>User agents SHOULD deprecate the <code>User-Agent</code> header in favor of the Client Hints model described in
this document. The header, however, is likely to be impossible to remove entirely in the near-term,
as existing sites' content negotiation code will continue to require its presence (see <a data-link-type="biblio" href="#biblio-rossi2015">[Rossi2015]</a> for a recent example of a new browser’s struggles in this area).</p>
   <p>One approach which might be advisable could be for each user agent to lock the value of its <code>User-Agent</code> header, ensuring backwards compatibility by maintaining the crufty declarations of
"like Gecko" and "AppleWebKit/537.36" on into eternity. This can ratchet over time, first freezing
the version number, then shifting platform and model information to something reasonably generic in
order to reduce the fingerprint the header provides.</p>
   <h3 class="heading settled" data-level="5.2" id="grease"><span class="secno">5.2. </span><span class="content">GREASE-like UA Strings</span><a class="self-link" href="#grease"></a></h3>
   <p>History has shown us that there are real incentives for user agents to lie about their branding
in order to thread the needle of sites' sniffing scripts. While I’m optimistic that we can reset
expectations around sniffing by freezing the thing that’s sniffed-upon today, and creating a sane 
set of options for developers, it’s likely that this is hopelessly naive. It’s reasonable to
ponder what we should do to encourage sniffing in the right way, if we believe it’s going to
happen one way or another.</p>
   <p>User agents may choose to model <code>UA</code> as a set, rather than a single entry. This could encourage
standardized processing of the <code>UA</code> string by 
Randomly including additional, intentionally incorrect, comma-separated entries with arbitrary
ordering (similar conceptually to <a data-link-type="biblio" href="#biblio-i-dietf-tls-grease">[I-D.ietf-tls-grease]</a>) could encourage standardized processing
if the <code>UA</code> string by servers, and reduce the chance that we ossify on a few required strings.
For example, Chrome 73’s <code>Sec-CH-UA</code> header might be <code>"Chrome 73", "NotBrowser 12"</code>, or <code>"BrowsingIsFun Version 12b", "Chrome 73"</code>, or something completely different.</p>
   <h3 class="heading settled" data-level="5.3" id="sec-ch"><span class="secno">5.3. </span><span class="content">The 'Sec-CH-' prefix</span><a class="self-link" href="#sec-ch"></a></h3>
   <p>Based on some discussion in https://github.com/w3ctag/design-reviews/issues/320, it seems
reasonable to forbid access to these headers from JavaScript, and demarcate them as
browser-controlled client hints so they can be documented and included in requests without
triggering CORS preflights. A <code>Sec-CH-</code> prefix seems like a viable approach, but this bit might
shift as the broader Client Hints discussions above coalesce into something more solid that lands
in specs.</p>
   <h2 class="heading settled" data-level="6" id="iana"><span class="secno">6. </span><span class="content">IANA Considerations</span><a class="self-link" href="#iana"></a></h2>
   <p>This document intends to define the <code>Sec-CH-Arch</code>, <code>Sec-CH-Model</code>, <code>Sec-CH-Platform</code>, and <code>Sec-CH-UA</code> HTTP request header fields, and register them in the permanent message header
field registry (<a data-link-type="biblio" href="#biblio-rfc3864">[RFC3864]</a>).</p>
   <p>It also intends to deprecate the <code>User-Agent</code> header field.</p>
   <h3 class="heading settled" data-level="6.1" id="iana-arch"><span class="secno">6.1. </span><span class="content">'Sec-CH-Arch' Header Field</span><a class="self-link" href="#iana-arch"></a></h3>
   <p>Header field name:</p>
   <dl>
    <dt data-md>Sec-CH-Arch
   </dl>
   <p>Applicable protocol:</p>
   <dl>
    <dt data-md>http
   </dl>
   <p>Status:</p>
   <dl>
    <dt data-md>standard
   </dl>
   <p>Author/Change controller:</p>
   <dl>
    <dt data-md>IETF
   </dl>
   <p>Specification document:</p>
   <dl>
    <dt data-md>this specification (<a href="#sec-ch-arch">§ 2.1 The 'Sec-CH-Arch' Header Field</a>)
   </dl>
   <h3 class="heading settled" data-level="6.2" id="iana-model"><span class="secno">6.2. </span><span class="content">'Sec-CH-Model' Header Field</span><a class="self-link" href="#iana-model"></a></h3>
   <p>Header field name:</p>
   <dl>
    <dt data-md>Sec-CH-Model
   </dl>
   <p>Applicable protocol:</p>
   <dl>
    <dt data-md>http
   </dl>
   <p>Status:</p>
   <dl>
    <dt data-md>standard
   </dl>
   <p>Author/Change controller:</p>
   <dl>
    <dt data-md>IETF
   </dl>
   <p>Specification document:</p>
   <dl>
    <dt data-md>this specification (<a href="#sec-ch-ua">§ 2.4 The 'Sec-CH-UA' Header Field</a>)
   </dl>
   <h3 class="heading settled" data-level="6.3" id="iana-platform"><span class="secno">6.3. </span><span class="content">'Sec-CH-Platform' Header Field</span><a class="self-link" href="#iana-platform"></a></h3>
   <p>Header field name:</p>
   <dl>
    <dt data-md>Sec-CH-Platform
   </dl>
   <p>Applicable protocol:</p>
   <dl>
    <dt data-md>http
   </dl>
   <p>Status:</p>
   <dl>
    <dt data-md>standard
   </dl>
   <p>Author/Change controller:</p>
   <dl>
    <dt data-md>IETF
   </dl>
   <p>Specification document:</p>
   <dl>
    <dt data-md>this specification (<a href="#sec-ch-platform">§ 2.3 The 'Sec-CH-Platform' Header Field</a>)
   </dl>
   <h3 class="heading settled" data-level="6.4" id="iana-ua"><span class="secno">6.4. </span><span class="content">'Sec-CH-UA' Header Field</span><a class="self-link" href="#iana-ua"></a></h3>
   <p>Header field name:</p>
   <dl>
    <dt data-md>Sec-CH-UA
   </dl>
   <p>Applicable protocol:</p>
   <dl>
    <dt data-md>http
   </dl>
   <p>Status:</p>
   <dl>
    <dt data-md>standard
   </dl>
   <p>Author/Change controller:</p>
   <dl>
    <dt data-md>IETF
   </dl>
   <p>Specification document:</p>
   <dl>
    <dt data-md>this specification (<a href="#sec-ch-ua">§ 2.4 The 'Sec-CH-UA' Header Field</a>)
   </dl>
   <h3 class="heading settled" data-level="6.5" id="iana-mobile"><span class="secno">6.5. </span><span class="content">'Sec-CH-Mobile' Header Field</span><a class="self-link" href="#iana-mobile"></a></h3>
   <p>Header field name:</p>
   <dl>
    <dt data-md>Sec-CH-Mobile
   </dl>
   <p>Applicable protocol:</p>
   <dl>
    <dt data-md>http
   </dl>
   <p>Status:</p>
   <dl>
    <dt data-md>standard
   </dl>
   <p>Author/Change controller:</p>
   <dl>
    <dt data-md>IETF
   </dl>
   <p>Specification document:</p>
   <dl>
    <dt data-md>this specification (<a href="#sec-ch-mobile">§ 2.5 The 'Sec-CH-Mobile' Header Field</a>)
   </dl>
   <h3 class="heading settled" data-level="6.6" id="iana-user-agent"><span class="secno">6.6. </span><span class="content">'User-Agent' Header Field</span><a class="self-link" href="#iana-user-agent"></a></h3>
   <p>Header field name:</p>
   <dl>
    <dt data-md>User-Agent
   </dl>
   <p>Applicable protocol:</p>
   <dl>
    <dt data-md>http
   </dl>
   <p>Status:</p>
   <dl>
    <dt data-md>deprecated
   </dl>
   <p>Author/Change controller:</p>
   <dl>
    <dt data-md>IETF
   </dl>
   <p>Specification document:</p>
   <dl>
    <dt data-md>this specification (<a href="#user-agent">§ 5.1 The 'User-Agent' Header</a>), and Section 5.5.3 of <a data-link-type="biblio" href="#biblio-rfc7231">[RFC7231]</a>
   </dl>
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  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#dom-navigatoruadata-architecture">architecture</a><span>, in §3</span>
   <li>
    brand
    <ul>
     <li><a href="#dom-navigatoruadata-brand">attribute for NavigatorUAData</a><span>, in §3</span>
     <li><a href="#user-agent-brand">dfn for user agent</a><span>, in §2</span>
    </ul>
   <li><a href="#request-client-hints-set">client-hints set</a><span>, in §4.2</span>
   <li><a href="#user-agent-full-version">full version</a><span>, in §2</span>
   <li><a href="#dom-navigatorua-getuseragent">getUserAgent()</a><span>, in §3.1</span>
   <li><a href="#user-agent-major-version">major version</a><span>, in §2</span>
   <li><a href="#dom-navigatoruadata-mobile">mobile</a><span>, in §3</span>
   <li><a href="#user-agent-mobileness">mobileness</a><span>, in §2</span>
   <li>
    model
    <ul>
     <li><a href="#dom-navigatoruadata-model">attribute for NavigatorUAData</a><span>, in §3</span>
     <li><a href="#user-agent-model">dfn for user agent</a><span>, in §2</span>
    </ul>
   <li><a href="#navigatorua">NavigatorUA</a><span>, in §3</span>
   <li><a href="#navigatoruadata">NavigatorUAData</a><span>, in §3</span>
   <li><a href="#dom-navigatoruadata-platform">platform</a><span>, in §3</span>
   <li><a href="#user-agent-platform-architecture">platform architecture</a><span>, in §2</span>
   <li><a href="#user-agent-platform-brand-and-version">platform brand and version</a><span>, in §2</span>
   <li><a href="#http-headerdef-sec-ch-arch">Sec-CH-Arch</a><span>, in §2.1</span>
   <li><a href="#http-headerdef-sec-ch-mobile">Sec-CH-Mobile</a><span>, in §2.5</span>
   <li><a href="#http-headerdef-sec-ch-model">Sec-CH-Model</a><span>, in §2.2</span>
   <li><a href="#http-headerdef-sec-ch-platform">Sec-CH-Platform</a><span>, in §2.3</span>
   <li><a href="#http-headerdef-sec-ch-ua">Sec-CH-UA</a><span>, in §2.4</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-arch-header-for-a-request">set-arch</a><span>, in §2.1</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-mobile-header-for-a-request">set-mobile</a><span>, in §2.5</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-model-header-for-a-request">set-model</a><span>, in §2.2</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-platform-header-for-a-request">set-platform</a><span>, in §2.3</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-arch-header-for-a-request">set the Sec-CH-Arch header for a request</a><span>, in §2.1</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-mobile-header-for-a-request">set the Sec-CH-Mobile header for a request</a><span>, in §2.5</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-model-header-for-a-request">set the Sec-CH-Model header for a request</a><span>, in §2.2</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-platform-header-for-a-request">set the Sec-CH-Platform header for a request</a><span>, in §2.3</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-ua-header-for-a-request">set the Sec-CH-UA header for a request</a><span>, in §2.4</span>
   <li><a href="#abstract-opdef-set-the-user-agent-metadata-for-a-request">set the user agent metadata for a request</a><span>, in §2.6</span>
   <li><a href="#abstract-opdef-set-the-sec-ch-ua-header-for-a-request">set-ua</a><span>, in §2.4</span>
   <li><a href="#dom-navigatoruadata-version">version</a><span>, in §3</span>
  </ul>
  <aside class="dfn-panel" data-for="term-for-header-list-contains">
   <a href="https://fetch.spec.whatwg.org/#header-list-contains">https://fetch.spec.whatwg.org/#header-list-contains</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-header-list-contains">2.6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-request-header-list">
   <a href="https://fetch.spec.whatwg.org/#concept-request-header-list">https://fetch.spec.whatwg.org/#concept-request-header-list</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-request-header-list">2.1. The 'Sec-CH-Arch' Header Field</a>
    <li><a href="#ref-for-concept-request-header-list①">2.2. The 'Sec-CH-Model' Header Field</a>
    <li><a href="#ref-for-concept-request-header-list②">2.3. The 'Sec-CH-Platform' Header Field</a>
    <li><a href="#ref-for-concept-request-header-list③">2.4. The 'Sec-CH-UA' Header Field</a>
    <li><a href="#ref-for-concept-request-header-list④">2.5. The 'Sec-CH-Mobile' Header Field</a>
    <li><a href="#ref-for-concept-request-header-list⑤">2.6. Integration with Fetch</a> <a href="#ref-for-concept-request-header-list⑥">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-request">
   <a href="https://fetch.spec.whatwg.org/#concept-request">https://fetch.spec.whatwg.org/#concept-request</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-request">2.1. The 'Sec-CH-Arch' Header Field</a>
    <li><a href="#ref-for-concept-request①">2.2. The 'Sec-CH-Model' Header Field</a>
    <li><a href="#ref-for-concept-request②">2.3. The 'Sec-CH-Platform' Header Field</a>
    <li><a href="#ref-for-concept-request③">2.4. The 'Sec-CH-UA' Header Field</a>
    <li><a href="#ref-for-concept-request④">2.5. The 'Sec-CH-Mobile' Header Field</a>
    <li><a href="#ref-for-concept-request⑤">4.2. Delegation</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-navigator">
   <a href="https://html.spec.whatwg.org/multipage/system-state.html#navigator">https://html.spec.whatwg.org/multipage/system-state.html#navigator</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-navigator">3. Interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-in-parallel">
   <a href="https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel">https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-in-parallel">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-section-3.3.4">
   <a href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.3.4">https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.3.4</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-section-3.3.4">2.5. The 'Sec-CH-Mobile' Header Field</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-section-3.1">
   <a href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.1">https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.1</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-section-3.1">2.4. The 'Sec-CH-UA' Header Field</a> <a href="#ref-for-section-3.1①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-section-3.3.3">
   <a href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.3.3">https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-3.3.3</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-section-3.3.3">2.1. The 'Sec-CH-Arch' Header Field</a>
    <li><a href="#ref-for-section-3.3.3①">2.2. The 'Sec-CH-Model' Header Field</a>
    <li><a href="#ref-for-section-3.3.3②">2.3. The 'Sec-CH-Platform' Header Field</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-">
   <a href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#">https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#</a><b>Referenced in:</b>
   <ul>
    <li><a href="#termref-for-">2.1. The 'Sec-CH-Arch' Header Field</a> <a href="#termref-for-">(2)</a>
    <li><a href="#termref-for-">2.2. The 'Sec-CH-Model' Header Field</a> <a href="#termref-for-">(2)</a>
    <li><a href="#termref-for-">2.3. The 'Sec-CH-Platform' Header Field</a> <a href="#termref-for-">(2)</a>
    <li><a href="#termref-for-">2.4. The 'Sec-CH-UA' Header Field</a> <a href="#termref-for-">(2)</a>
    <li><a href="#termref-for-">2.5. The 'Sec-CH-Mobile' Header Field</a> <a href="#termref-for-">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-list-append">
   <a href="https://infra.spec.whatwg.org/#list-append">https://infra.spec.whatwg.org/#list-append</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-list-append">2.4. The 'Sec-CH-UA' Header Field</a> <a href="#ref-for-list-append①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-string-concatenate">
   <a href="https://infra.spec.whatwg.org/#string-concatenate">https://infra.spec.whatwg.org/#string-concatenate</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-string-concatenate">2.4. The 'Sec-CH-UA' Header Field</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-list-contain">
   <a href="https://infra.spec.whatwg.org/#list-contain">https://infra.spec.whatwg.org/#list-contain</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-list-contain">2.1. The 'Sec-CH-Arch' Header Field</a>
    <li><a href="#ref-for-list-contain①">2.2. The 'Sec-CH-Model' Header Field</a>
    <li><a href="#ref-for-list-contain②">2.3. The 'Sec-CH-Platform' Header Field</a>
    <li><a href="#ref-for-list-contain③">2.4. The 'Sec-CH-UA' Header Field</a>
    <li><a href="#ref-for-list-contain④">2.5. The 'Sec-CH-Mobile' Header Field</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-a-new-promise">
   <a href="https://www.w3.org/2001/tag/doc/promises-guide/#a-new-promise">https://www.w3.org/2001/tag/doc/promises-guide/#a-new-promise</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-a-new-promise">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-idl-DOMString">
   <a href="https://heycam.github.io/webidl/#idl-DOMString">https://heycam.github.io/webidl/#idl-DOMString</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-idl-DOMString">3. Interface</a> <a href="#ref-for-idl-DOMString①">(2)</a> <a href="#ref-for-idl-DOMString②">(3)</a> <a href="#ref-for-idl-DOMString③">(4)</a> <a href="#ref-for-idl-DOMString④">(5)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-Exposed">
   <a href="https://heycam.github.io/webidl/#Exposed">https://heycam.github.io/webidl/#Exposed</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-Exposed">3. Interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-SecureContext">
   <a href="https://heycam.github.io/webidl/#SecureContext">https://heycam.github.io/webidl/#SecureContext</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-SecureContext">3. Interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-idl-boolean">
   <a href="https://heycam.github.io/webidl/#idl-boolean">https://heycam.github.io/webidl/#idl-boolean</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-idl-boolean">3. Interface</a>
   </ul>
  </aside>
  <h3 class="no-num no-ref heading settled" id="index-defined-elsewhere"><span class="content">Terms defined by reference</span><a class="self-link" href="#index-defined-elsewhere"></a></h3>
  <ul class="index">
   <li>
    <a data-link-type="biblio">[FETCH]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-header-list-contains" style="color:initial">contains</span>
     <li><span class="dfn-paneled" id="term-for-concept-request-header-list" style="color:initial">header list</span>
     <li><span class="dfn-paneled" id="term-for-concept-request" style="color:initial">request</span>
    </ul>
   <li>
    <a data-link-type="biblio">[HTML]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-navigator" style="color:initial">Navigator</span>
     <li><span class="dfn-paneled" id="term-for-in-parallel" style="color:initial">in parallel</span>
    </ul>
   <li>
    <a data-link-type="biblio">[I-D.ietf-httpbis-header-structure]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-section-3.3.4" style="color:initial">boolean</span>
     <li><span class="dfn-paneled" id="term-for-section-3.1" style="color:initial">list</span>
     <li><span class="dfn-paneled" id="term-for-section-3.3.3" style="color:initial">string</span>
     <li><span class="dfn-paneled" id="term-for-" style="color:initial">structured header</span>
    </ul>
   <li>
    <a data-link-type="biblio">[INFRA]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-list-append" style="color:initial">append</span>
     <li><span class="dfn-paneled" id="term-for-string-concatenate" style="color:initial">concatenation</span>
     <li><span class="dfn-paneled" id="term-for-list-contain" style="color:initial">contain</span>
    </ul>
   <li>
    <a data-link-type="biblio">[promises-guide]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-a-new-promise" style="color:initial">a new promise</span>
    </ul>
   <li>
    <a data-link-type="biblio">[WebIDL]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-idl-DOMString" style="color:initial">DOMString</span>
     <li><span class="dfn-paneled" id="term-for-Exposed" style="color:initial">Exposed</span>
     <li><span class="dfn-paneled" id="term-for-SecureContext" style="color:initial">SecureContext</span>
     <li><span class="dfn-paneled" id="term-for-idl-boolean" style="color:initial">boolean</span>
    </ul>
  </ul>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-fetch">[FETCH]
   <dd>Anne van Kesteren. <a href="https://fetch.spec.whatwg.org/">Fetch Standard</a>. Living Standard. URL: <a href="https://fetch.spec.whatwg.org/">https://fetch.spec.whatwg.org/</a>
   <dt id="biblio-html">[HTML]
   <dd>Anne van Kesteren; et al. <a href="https://html.spec.whatwg.org/multipage/">HTML Standard</a>. Living Standard. URL: <a href="https://html.spec.whatwg.org/multipage/">https://html.spec.whatwg.org/multipage/</a>
   <dt id="biblio-i-dietf-httpbis-header-structure">[I-D.ietf-httpbis-header-structure]
   <dd>Mark Nottingham; Poul-Henning Kamp. <a href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure">Structured Headers for HTTP</a>. ID. URL: <a href="https://tools.ietf.org/html/draft-ietf-httpbis-header-structure">https://tools.ietf.org/html/draft-ietf-httpbis-header-structure</a>
   <dt id="biblio-infra">[INFRA]
   <dd>Anne van Kesteren; Domenic Denicola. <a href="https://infra.spec.whatwg.org/">Infra Standard</a>. Living Standard. URL: <a href="https://infra.spec.whatwg.org/">https://infra.spec.whatwg.org/</a>
   <dt id="biblio-promises-guide">[PROMISES-GUIDE]
   <dd>Domenic Denicola. <a href="https://www.w3.org/2001/tag/doc/promises-guide">Writing Promise-Using Specifications</a>. 9 November 2018. TAG Finding. URL: <a href="https://www.w3.org/2001/tag/doc/promises-guide">https://www.w3.org/2001/tag/doc/promises-guide</a>
   <dt id="biblio-webidl">[WebIDL]
   <dd>Boris Zbarsky. <a href="https://heycam.github.io/webidl/">Web IDL</a>. URL: <a href="https://heycam.github.io/webidl/">https://heycam.github.io/webidl/</a>
  </dl>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-facebookyearclass">[FacebookYearClass]
   <dd>Chris Marra; Daniel Weaver. <a href="https://code.fb.com/android/year-class-a-classifiation-system-for-android/">Year class: A classification system for Android</a>. URL: <a href="https://code.fb.com/android/year-class-a-classifiation-system-for-android/">https://code.fb.com/android/year-class-a-classifiation-system-for-android/</a>
   <dt id="biblio-i-dietf-httpbis-client-hints">[I-D.ietf-httpbis-client-hints]
   <dd>Ilya Grigorik. <a href="https://tools.ietf.org/html/draft-ietf-httpbis-client-hints">HTTP Client Hints</a>. ID. URL: <a href="https://tools.ietf.org/html/draft-ietf-httpbis-client-hints">https://tools.ietf.org/html/draft-ietf-httpbis-client-hints</a>
   <dt id="biblio-i-dietf-tls-grease">[I-D.ietf-tls-grease]
   <dd>David Benjamin. <a href="https://tools.ietf.org/html/draft-ietf-tls-grease">Applying GREASE to TLS Extensibility</a>. ID. URL: <a href="https://tools.ietf.org/html/draft-ietf-tls-grease">https://tools.ietf.org/html/draft-ietf-tls-grease</a>
   <dt id="biblio-janc2014">[Janc2014]
   <dd>Artur Janc; Michal Zalweski. <a href="https://dev.chromium.org/Home/chromium-security/client-identification-mechanisms#TOC-Browser-level-fingerprints">Technical analysis of client identification mechanisms</a>. URL: <a href="https://dev.chromium.org/Home/chromium-security/client-identification-mechanisms#TOC-Browser-level-fingerprints">https://dev.chromium.org/Home/chromium-security/client-identification-mechanisms#TOC-Browser-level-fingerprints</a>
   <dt id="biblio-rfc3864">[RFC3864]
   <dd>G. Klyne; M. Nottingham; J. Mogul. <a href="https://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>. September 2004. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc3864">https://tools.ietf.org/html/rfc3864</a>
   <dt id="biblio-rfc7231">[RFC7231]
   <dd>R. Fielding, Ed.; J. Reschke, Ed.. <a href="https://httpwg.org/specs/rfc7231.html">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>. June 2014. Proposed Standard. URL: <a href="https://httpwg.org/specs/rfc7231.html">https://httpwg.org/specs/rfc7231.html</a>
   <dt id="biblio-rossi2015">[Rossi2015]
   <dd><a href="https://channel9.msdn.com/Events/WebPlatformSummit/2015/The-Microsoft-Edge-Rendering-Engine-that-makes-the-Web-just-work#time=9m45s">The Microsoft Edge Rendering Engine that makes the Web just work</a>. URL: <a href="https://channel9.msdn.com/Events/WebPlatformSummit/2015/The-Microsoft-Edge-Rendering-Engine-that-makes-the-Web-just-work#time=9m45s">https://channel9.msdn.com/Events/WebPlatformSummit/2015/The-Microsoft-Edge-Rendering-Engine-that-makes-the-Web-just-work#time=9m45s</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="idl-index"><span class="content">IDL Index</span><a class="self-link" href="#idl-index"></a></h2>
<pre class="idl highlight def">[<a class="idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#Exposed" id="ref-for-Exposed①"><c- g>Exposed</c-></a>=<c- n>Window</c->]
<c- b>interface</c-> <a href="#navigatoruadata"><code><c- g>NavigatorUAData</c-></code></a> {
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString⑤"><c- b>DOMString</c-></a> <a data-readonly data-type="DOMString" href="#dom-navigatoruadata-brand"><code><c- g>brand</c-></code></a>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString①①"><c- b>DOMString</c-></a> <a data-readonly data-type="DOMString" href="#dom-navigatoruadata-version"><code><c- g>version</c-></code></a>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString②①"><c- b>DOMString</c-></a> <a data-readonly data-type="DOMString" href="#dom-navigatoruadata-platform"><code><c- g>platform</c-></code></a>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString③①"><c- b>DOMString</c-></a> <a data-readonly data-type="DOMString" href="#dom-navigatoruadata-architecture"><code><c- g>architecture</c-></code></a>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString④①"><c- b>DOMString</c-></a> <a data-readonly data-type="DOMString" href="#dom-navigatoruadata-model"><code><c- g>model</c-></code></a>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-boolean" id="ref-for-idl-boolean①"><c- b>boolean</c-></a> <a data-readonly data-type="boolean" href="#dom-navigatoruadata-mobile"><code><c- g>mobile</c-></code></a>;
};
<c- b>interface</c-> <c- b>mixin</c-> <a href="#navigatorua"><code><c- g>NavigatorUA</c-></code></a> {
  [<a class="idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#SecureContext" id="ref-for-SecureContext①"><c- g>SecureContext</c-></a>] <c- b>Promise</c->&lt;<a class="n" data-link-type="idl-name" href="#navigatoruadata" id="ref-for-navigatoruadata②"><c- n>NavigatorUAData</c-></a>> <a class="idl-code" data-link-type="method" href="#dom-navigatorua-getuseragent" id="ref-for-dom-navigatorua-getuseragent①"><c- g>getUserAgent</c-></a>();
};
<a class="n" data-link-type="idl-name" href="https://html.spec.whatwg.org/multipage/system-state.html#navigator" id="ref-for-navigator①"><c- n>Navigator</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="#navigatorua" id="ref-for-navigatorua①"><c- n>NavigatorUA</c-></a>;


</pre>
  <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
  <div style="counter-reset:issue">
   <div class="issue"> Perhaps <code>Sec-CH-Mobile</code> is enough, and we don’t need to expose the model?<a href="#issue-a1588654"> ↵ </a></div>
   <div class="issue"> Should we split the version out into a separate <code>Sec-CH-UA-Version</code> header? Or keep it here? <a href="https://github.com/wicg/ua-client-hints/issues/7">&lt;https://github.com/wicg/ua-client-hints/issues/7></a><a href="#issue-af2ff260"> ↵ </a></div>
   <div class="issue"> Provide a method to only access the UA’s major version.<a href="#issue-611030b9"> ↵ </a></div>
   <div class="issue"> Fetch integration of Accept-CH opt-in, and the definition of a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request">request</a>'s <span>client-hints set</span>. <a href="https://github.com/whatwg/fetch/issues/773">&lt;https://github.com/whatwg/fetch/issues/773></a><a href="#issue-b875ecdd"> ↵ </a></div>
   <div class="issue"> HTML integration of Accept-CH-Lifetime and the ACHL cache. <a href="https://github.com/whatwg/html/issues/3774">&lt;https://github.com/whatwg/html/issues/3774></a><a href="#issue-ff94095d"> ↵ </a></div>
   <div class="issue"> Adding new CH features to the CH list in Fetch <a href="https://github.com/whatwg/fetch/issues/725">&lt;https://github.com/whatwg/fetch/issues/725></a><a href="#issue-ad33bb81"> ↵ </a></div>
   <div class="issue"> 3rd-party opt-in <a href="https://github.com/whatwg/fetch/issues/811">&lt;https://github.com/whatwg/fetch/issues/811></a><a href="#issue-4794e88a"> ↵ </a></div>
   <div class="issue"> 3rd-party opt-in <a href="https://github.com/wicg/feature-policy/issues/220">&lt;https://github.com/wicg/feature-policy/issues/220></a><a href="#issue-4794e88a①"> ↵ </a></div>
   <div class="issue"> These are all out of date. Yoav will fix them.<a href="#issue-4b5acc88"> ↵ </a></div>
  </div>
  <aside class="dfn-panel" data-for="user-agent-brand">
   <b><a href="#user-agent-brand">#user-agent-brand</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-user-agent-brand">2.4. The 'Sec-CH-UA' Header Field</a>
    <li><a href="#ref-for-user-agent-brand①">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="user-agent-major-version">
   <b><a href="#user-agent-major-version">#user-agent-major-version</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-user-agent-major-version">2.4. The 'Sec-CH-UA' Header Field</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="user-agent-full-version">
   <b><a href="#user-agent-full-version">#user-agent-full-version</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-user-agent-full-version">2.4. The 'Sec-CH-UA' Header Field</a>
    <li><a href="#ref-for-user-agent-full-version①">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="user-agent-platform-brand-and-version">
   <b><a href="#user-agent-platform-brand-and-version">#user-agent-platform-brand-and-version</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-user-agent-platform-brand-and-version">2.3. The 'Sec-CH-Platform' Header Field</a>
    <li><a href="#ref-for-user-agent-platform-brand-and-version①">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="user-agent-platform-architecture">
   <b><a href="#user-agent-platform-architecture">#user-agent-platform-architecture</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-user-agent-platform-architecture">2.1. The 'Sec-CH-Arch' Header Field</a>
    <li><a href="#ref-for-user-agent-platform-architecture①">3.1. Processing model</a>
    <li><a href="#ref-for-user-agent-platform-architecture②">4.3. Access Restrictions</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="user-agent-model">
   <b><a href="#user-agent-model">#user-agent-model</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-user-agent-model">2.2. The 'Sec-CH-Model' Header Field</a>
    <li><a href="#ref-for-user-agent-model①">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="user-agent-mobileness">
   <b><a href="#user-agent-mobileness">#user-agent-mobileness</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-user-agent-mobileness">2.5. The 'Sec-CH-Mobile' Header Field</a>
    <li><a href="#ref-for-user-agent-mobileness①">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="abstract-opdef-set-the-sec-ch-arch-header-for-a-request">
   <b><a href="#abstract-opdef-set-the-sec-ch-arch-header-for-a-request">#abstract-opdef-set-the-sec-ch-arch-header-for-a-request</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-abstract-opdef-set-the-sec-ch-arch-header-for-a-request">2.6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="abstract-opdef-set-the-sec-ch-model-header-for-a-request">
   <b><a href="#abstract-opdef-set-the-sec-ch-model-header-for-a-request">#abstract-opdef-set-the-sec-ch-model-header-for-a-request</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-abstract-opdef-set-the-sec-ch-model-header-for-a-request">2.6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="abstract-opdef-set-the-sec-ch-platform-header-for-a-request">
   <b><a href="#abstract-opdef-set-the-sec-ch-platform-header-for-a-request">#abstract-opdef-set-the-sec-ch-platform-header-for-a-request</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-abstract-opdef-set-the-sec-ch-platform-header-for-a-request">2.6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="abstract-opdef-set-the-sec-ch-ua-header-for-a-request">
   <b><a href="#abstract-opdef-set-the-sec-ch-ua-header-for-a-request">#abstract-opdef-set-the-sec-ch-ua-header-for-a-request</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-abstract-opdef-set-the-sec-ch-ua-header-for-a-request">2.6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="abstract-opdef-set-the-sec-ch-mobile-header-for-a-request">
   <b><a href="#abstract-opdef-set-the-sec-ch-mobile-header-for-a-request">#abstract-opdef-set-the-sec-ch-mobile-header-for-a-request</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-abstract-opdef-set-the-sec-ch-mobile-header-for-a-request">2.6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="navigatoruadata">
   <b><a href="#navigatoruadata">#navigatoruadata</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-navigatoruadata">3. Interface</a>
    <li><a href="#ref-for-navigatoruadata①">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-navigatoruadata-brand">
   <b><a href="#dom-navigatoruadata-brand">#dom-navigatoruadata-brand</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-navigatoruadata-brand">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-navigatoruadata-version">
   <b><a href="#dom-navigatoruadata-version">#dom-navigatoruadata-version</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-navigatoruadata-version">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-navigatoruadata-platform">
   <b><a href="#dom-navigatoruadata-platform">#dom-navigatoruadata-platform</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-navigatoruadata-platform">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-navigatoruadata-architecture">
   <b><a href="#dom-navigatoruadata-architecture">#dom-navigatoruadata-architecture</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-navigatoruadata-architecture">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-navigatoruadata-model">
   <b><a href="#dom-navigatoruadata-model">#dom-navigatoruadata-model</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-navigatoruadata-model">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-navigatoruadata-mobile">
   <b><a href="#dom-navigatoruadata-mobile">#dom-navigatoruadata-mobile</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-navigatoruadata-mobile">3.1. Processing model</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="navigatorua">
   <b><a href="#navigatorua">#navigatorua</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-navigatorua">3. Interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-navigatorua-getuseragent">
   <b><a href="#dom-navigatorua-getuseragent">#dom-navigatorua-getuseragent</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-navigatorua-getuseragent">3. Interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="request-client-hints-set">
   <b><a href="#request-client-hints-set">#request-client-hints-set</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-request-client-hints-set">2.1. The 'Sec-CH-Arch' Header Field</a>
    <li><a href="#ref-for-request-client-hints-set①">2.2. The 'Sec-CH-Model' Header Field</a>
    <li><a href="#ref-for-request-client-hints-set②">2.3. The 'Sec-CH-Platform' Header Field</a>
    <li><a href="#ref-for-request-client-hints-set③">2.4. The 'Sec-CH-UA' Header Field</a>
    <li><a href="#ref-for-request-client-hints-set④">2.5. The 'Sec-CH-Mobile' Header Field</a>
   </ul>
  </aside>
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